DE69105913T2 - Method and apparatus for sheathing and holding a core through a sheath of composite material. - Google Patents

Abstract

According to the invention, the method consists in: - producing the said composite sleeve (4) whose internal cross-section is, before insertion of the core (6), less than the external cross-section of the core. - aligning the said core (6) and the said sleeve (4) by substantially centring them one with respect to the other, - exerting a prior traction force on the said core (6) so as to provide a suitable sealing between the contacting portions of each of the ends of the core (6) and of the sleeve (4), - injecting a pressurised fluid into the said sleeve (4) and in the direction of the said core (6) so as progressively to increase, by moving along the axial direction, the cross-section of the sleeve (4) and, possibly, to reduce the cross-section of the core (6) and, simultaneously, continuing to exert a traction force on the said core (6) so as gradually to insert the said core (6) into the said sleeve (4), - stopping the injection of fluid after complete insertion of the core (6) into the sleeve (4), - cutting off the redundant portion of the sleeve held by the end-fitting (1). <IMAGE>

Description

The invention relates to a method for encasing and maintaining under pressure a core by means of an outer casing.

The process is intended for the manufacture of devices requiring an external enclosure to contain a central core. The external enclosure must therefore exert a radial pressure on the central core to hold it in place and, in some cases, limit deformations of the core, which may be subject to forces tending to expand the enclosure.

In particular, the invention relates to a process that makes it possible to encase and maintain under pressure a core subjected to considerable stresses, these stresses being able to be radial and/or longitudinal and/or circumferential and thus requiring a corresponding internal depressurization (for example of the order of 300 - 1000 bar), the core having a length that can reach several meters. The process according to the invention is thus intended to make it possible to manufacture sewer pipes subjected to internal pressures and/or radial dynamic stresses and/or longitudinal and/or circumferential stresses that are quite considerable.

The method according to the invention is also designed so that a core can be covered by a sheath in the case where the core and/or the sheath are made of materials which have low thermal expansion coefficients or where sheathing cannot be produced by thermal expansion phenomena.

The process according to the invention is particularly applicable to the coating of a metallic or non-metallic core by an outer coating of non-metallic material, the core and this coating being able to have any shape. More particularly, this coating can be made of composite material and can be obtained by thread-like winding of fibers impregnated with a binding polymerizable material.

Processes for wrapping with a bonded material are already known, in which a thread-like winding is made directly on the object to be wrapped. In particular, patent application EP-A-0 344 025, filed in the name of the applicant, describes a process which uses the technique of thread-like winding to produce vessels for storing fluids under pressure. The effectiveness of the wrapping only occurs when the container is subjected to internal pressure.

The methods by which a filamentary winding is produced directly on the object to be wrapped have limitations and shortcomings.

Since the composite material which makes up the sheath is composed of fibres bound together by a thermosetting plastic, polymerisation of the composite material is carried out after winding the fibres pre-impregnated with plastic or after impregnating the wound fibres in the dry state. This (polymerisation) is carried out by increasing the temperature in an oven. It is noted that the tension of the fibres during winding must be lower than the tensile strength of the fibres. In addition, the tension obtained during polymerization. Consequently, the residual tension of the fibers can only be much lower than the limit of the tensile strength of the fibers. The coating processes in which a thread-like winding is made directly on the object to be coated cannot therefore be suitable for cores which are subjected to relatively large radial and/or longitudinal and/or circumferential forces and which require corresponding holding under internal pressure.

On the other hand, it is possible to consider applying this type of process with thermoplastic composite materials in such a way as to avoid the polymerization step. In this case, the winding is made with fibers impregnated with a heated thermoplastic binder.

It is noted that such a process also has disadvantages. The bonding of the thermoplastic fibers by heating is achieved as they are applied to the object to be coated. Consequently, if it is possible to control the tension of the fibers at the time of their application, it is difficult to adjust the tension of the fibers in the layering of the layers. As a result, the limitations of implementation are significant and prevent the coatings obtained from being optimized. Optimization would in fact require a very complex calculation, on the one hand because the first layers of fibers relax in the case of a compressible core, which makes it difficult to control the pressure, and on the other hand because the circulation and distribution of the resin are difficult to control. Therefore, the number of fiber layers provided to obtain the desired coating pressure can be considerably higher than the number theoretically necessary. Furthermore, thermoplastic materials are still not widely used, and the most efficient of them are very complex.

A process is known which makes it possible to fix a substantially cylindrical core subjected to considerable internal pressures without creating a filamentary sheath directly on the core. In this process, the core is placed in an outer sheath which is also substantially cylindrical and preferably non-metallic. The space between the core and the outer sheath is filled with a fluid under pressure. The pressure of the fluid is controlled by pistons. The core is thus held in place by the fluid under pressure which surrounds it. The outer sheath can be made of composite material and the fluid under pressure can be a synthetic resin, possibly hardened, the pressure being maintained during its hardening.

This method allows to avoid the disadvantages associated with the realization of a direct thread-like winding, but it presents other (disadvantages).

In fact, in order to keep relatively long cores under pressure and therefore obtain a good distribution of the fluid along the core, it is necessary to provide a fairly significant clearance between the external casing and the core and/or several injection points of the fluid under pressure. The presence of a significant clearance for the injection of fluid, to which is added the increase in the diameter of the external casing under the effect of the internal pressure, results in relatively significant fluid thicknesses between the casing and the core. In the case where the fluid under pressure is a synthetic resin, a drop in pressure may occur due to the temporal creep of the synthetic resin after polymerization. In addition, there may be a risk of destruction of the synthetic resin layer if its thickness is too considerable. The need to provide several injection points of synthetic resin imposes conditions on the realization of a fluid circuit between the core and the external casing. The presence of this circuit may be a hindrance to certain types of use.

A process is also known which allows a core to be held in an external metal casing. This process consists in forcibly inserting a core into a metal casing whose internal diameter is smaller than the external diameter of the core. Very considerable tensile forces must be exerted on the external casing. This process cannot therefore be used if the external casing is made of a composite material. In fact, the tensile stresses cause deterioration of the composite material forming the casing by separating the fibres when the core is forced in.

Finally, European patent application 212130 relates to the enveloping of a tubular part by a reinforcing cylinder made of composite material, in which the expansion of the reinforcing cylinder and the compression of the tubular part are obtained by pressurising it by means of a fluid.

The tubular part is introduced into the reinforcement cylinder in the following way. The fluid under pressure, in particular a liquid, is introduced into a mounting housing by means of connecting holes. This fluid reduces the diameter of the tubular part, widening the inner diameter of the reinforcement cylinder. Since the inner diameter of the reinforcement cylinder is larger than the outer diameter of the tubular part, a control rod is acted upon so that the reinforcement cylinder encloses the tubular part. After the tubular part has been introduced into the reinforcement cylinder, the supply of the fluid under pressure is stopped. The tubular part expands and the diameter of the reinforcement cylinder is reduced so that it rests firmly on the tubular part with the desired tension.

The procedure of expanding the reinforcing cylinder by compressing the tubular part and then inserting this tubular part into the reinforcing cylinder requires a very complicated device. In order to obtain the compression effect over the entire length, this device requires complex adjustments that are very difficult to implement and, moreover, are not described in the publication.

This solution does not correspond to the possibility of producing long-length pipes at a reduced cost and, in addition, presents additional problems of implementation, such as the tightness of a composite material casing which does not have very precise measurement tolerances. The working equipment is very difficult to manufacture. In addition, this publication does not describe how the problems of tightness are solved when implementing the process. In fact, in this device there is an incompatibility between centering, tightness and expansion due to the type of clamping chosen, which is linked to the centering. The reduction of the diameter of the tubular part at the level of the clamps during assembly, as well as the increase of the diameter of the reinforcing cylinder clamped during assembly, appear particularly difficult and in any case not very precise. From the fact that if there is tightness, centering is given, and if there is centering, there can be no expansion or contraction, one does not see how one can simultaneously solve the problems of centering, tightness, expansion and contraction.

The invention is based on the object of eliminating these disadvantages by proposing a method for encasing and holding a core under pressure by a sheath made of composite material, wherein the sheath made of composite material has an internal cross-section which the insertion of the core is smaller than the inner portion of the core, so as to avoid any sliding of the core with respect to the outer casing under significant instantaneous stresses, which may be radial and/or longitudinal and/or circumferential.

According to the invention, the method consists in:

Holding one of the ends of the enclosure by means of a base;

aligning the core and the cladding at their other end, by substantially centering them with respect to each other;

applying a preliminary tensile force to the core in the direction of the sheath and, in the case of a hollow and axially open core, providing tight closure means such that an adequate seal is ensured between the contact area of each of the ends of the core and the sheath;

injecting a fluid under pressure into the casing and towards the core to create a pressure between the core and the casing and to progressively increase the cross-section of the casing in the axial direction and possibly reduce the cross-section of the core, and at the same time continuously exerting a tensile force on the core such that the core is gradually introduced into the casing;

Stop injecting after the core has been fully inserted into the casing;

Cutting off the excess section of the casing that is held by the base.

This process therefore has the advantage of allowing the expansion of the previously produced outer casing as the core is inserted into the casing and/or the compression of the core. The process therefore uses the expansion property of the core made of composite material and/or the compressibility of the core to allow the core to be inserted into the casing. This process therefore makes it possible to envelop a core with an outer casing and to keep it under pressure without having to resort to thermal expansion phenomena. to resort to.

Advantageously, the method further comprises the step of creating, prior to aligning and centering the casing and the core, complementary frustoconical shapes at the points of contact, with a frustoconical shape at the ends of the casing and a frustoconical shape at the ends of the core, such that the centering and the appropriate tightness between the core and the casing are achieved by mutual engagement of the conical shapes.

The shell and the outer surface of the core can have any shape.

Preferably, the inner surface and/or the outer surface of the casing have a surface of revolution. In fact, the injection of the fluid causes a uniform expansion of the outer casing and it is not deformed.

In the present application, the fact that the casing is not subject to deformation by injecting the fluid under pressure means that at every point the cross-section of the casing before and after the injection of the fluid are homothetic (similar).

Thus, if the casing is not a body of revolution, it may be necessary to keep the casing under pressure during the injection of the fluid in order to limit its deformation.

The outer surface of the core can also be a surface of revolution.

In particular, at least one of these surfaces may be cylindrical. This is particularly the case where the cladding and/or the core are tubes.

In particular, at least one of these surfaces may be truncated cone-shaped. It is understandable that this special shape facilitates the insertion of the core into the outer casing.

Preferably, the inner surface and the outer surface of the core are similar. This embodiment is advantageous because in this case the coating is effected by the outer coating itself and not partially by the injected fluid.

Therefore, if the inner surface of the cladding and the outer surface of the core are not similar, it may be advantageous to cladding the core prior to aligning the core and the cladding in such a way that the inner surface of the cladding and the outer surface of the core are similar.

Preferably, the fluid under pressure which is injected is a lubricating fluid to facilitate the introduction of the core into the casing.

In particular, a polymerizable synthetic resin can be chosen as the lubricating fluid.

In this case, after completely inserting the core into the casing and stopping the injection of the fluid under pressure, the process may continue to consist in polymerizing the remaining plastic mass present between the inner surface of the casing and the outer surface of the core. This additional process allows the core to be ensured to adhere to the inner surface of the composite material casing.

The outer surface of the composite material can be obtained by any suitable method. In particular, it can be obtained by rolling pre-impregnated fibers of polymerizable synthetic resin onto a support and then polymerizing be obtained.

The sheath comprises at least one layer of rolled fibers. The thickness of the sheath can be variable. This is advantageous in the case where the core is not subjected to internal stresses that are identical from one part of the core to another. For example, the core can be subjected to an internal pressure that is more significant than usual in a specific zone, in which case the outer sheath is designed to have a greater thickness in this zone in order to compensate for the localized excess pressure.

For the same purpose, the casing can be made of different materials depending on the internal pressure of the corresponding zone and the core.

The fibres used to manufacture the sheath are organic fibres or mineral fibres with high mechanical resistance such as carbon fibres, glass fibres, boron or aramid fibres.

The invention further relates to a device for carrying out the inventive method.

This device comprises a sliding rod of smaller diameter than the cross-section of the casing, which is intended to be connected to the core and arranged in the casing so as to form a gap between the outer surface of the rod and the inner surface of the casing. The rod is pierced with at least one longitudinal channel and at least one radial channel, the longitudinal channel communicating with the gap via the radial channel. This device further comprises a system ensuring the tightness between the casing and the rod, means for exerting a tensile force on the rod with respect to the casing and a device for dispensing a fluid under pressure. The latter device is connected to the longitudinal channel(s) to allow the introduction of fluid under pressure into the space between the outer surface of the rod and the inner surface of the casing.

The invention will be better understood and further objects, advantages and features of the invention will appear more clearly upon reading the following description of preferred embodiments, with reference to the drawings in which:

- Figure 1 shows a device for carrying out the method according to the invention, a core and an outer casing before the introduction of the core into the outer casing,

- Fig. 2 shows the device, the core and the outer casing according to Fig. 1 during the introduction of the core, in which the outer casing is provided with an outer calibration stiffening tube of the core,

- Fig. 3 is a view similar to Fig. 1 at the end of the introduction of the core into the outer casing,

- Fig. 4 is a view similar to Fig. 1, with the core fully inserted into the outer casing, and

- Figure 5 shows a specific example in which the core is covered by the sheath according to a method according to the invention.

The elements common to the different figures are designated by the same reference numerals.

With reference to Fig. 1, the device which allows the method according to the invention to be carried out comprises a first device 1 or a base which is hollow and has a substantially cylindrical external shape. This base 1 comprises a first part 2, the inner surface of which is substantially cylindrical, and a second part 3, whose inner surface forms a taper 3A. This taper 3A allows the introduction and retention of an outer casing 4 made of composite material in the base 1.

The device further comprises a rod 5. This rod is essentially cylindrical and its diameter is smaller than the inner diameter of the first part 2 of the base 1 and that of the casing 4 made of composite material. A gap 10 is thus formed between the inner surface of the movable rod 5 and the inner surface of the outer casing 4 and of the base 1.

This rod is intended to be connected to a core 6, for example made of metal, which is to be introduced into the casing 4 made of composite material according to the method according to the invention. Such connecting means are known to the person skilled in the art and are not described in detail here.

As shown in Fig. 1, the rod 5 is perforated with at least one longitudinal channel 7 and one radial channel 8. The longitudinal channel 7 is connected to the intermediate space 10 via the radial channel 8.

This device further comprises a device for producing a fluid under pressure. This device is mounted at the end of the longitudinal channel 7 which is located on the side opposite the core and is not shown in the figures.

The base also ensures tightness with the rod 5 thanks to a system 9 arranged at the level of the first part 2 of the base 1 and which consists in particular of gaskets.

The rod 5 is designed to be able to slide inside the casing 4 and the base 1. Accordingly, a device, not shown in the figure, is provided, which allows the rod to move substantially along the axis of the outer casing 4 and the base 1.

According to a process according to the invention, the outer casing made of composite material is manufactured in advance. Various processes are known which make it possible to obtain an outer casing made of composite material.

Reference can be made in particular to patent FR-2 198 817, which relates to a process for obtaining hollow cylindrical bodies. This process uses a mold with a horizontal axis that rotates at a given speed, which determines a centrifugal acceleration greater than 1 g. Thus, all the materials that are introduced into the interior of the mold by means of devices and that spread axially and longitudinally within it are distributed in cylindrical coaxial layers with the mold and with each other. The process therefore consists in forming several different layers, which can be composed of thermosetting plastic and reinforcing fibers, or of thermosetting plastic and an inert material. This process makes it possible to obtain hollow cylindrical bodies whose dimensions of the internal surface cannot be fully controlled. It is therefore only usable in the case where the method according to the present invention can be carried out with such bodies as an outer casing. This is particularly the case when the core is compressible.

It is also possible to use methods that use the technique of filament winding. According to this technique, a filament network can be formed by winding individual filaments impregnated with plastic onto a mandrel in a helical manner. It is also possible to wind the filaments longitudinally and circumferentially. In this case, it is possible to arrange a ring with teeth at each end of a mandrel support. The filaments can then be wound longitudinally are wound continuously, the fibres being turned around by the teeth carried by the rings. In this way, a longitudinal fibre or a cloth of longitudinal fibres goes around a tooth and then, following a small circumferential projection, reaches another adjacent tooth, goes around it and turns around it to spread out longitudinally over the mandrel. Such longitudinal winding can be followed by circumferential winding. Insofar as the wound fibres are pre-impregnated with plastic, it is also sufficient to carry out polymerisation to obtain a composite material covering. Insofar as, as we shall see in more detail in the following description, one of the process steps according to the invention consists in radially expanding the outer sheath of composite material and doing so continuously in the axial direction, that is to say as the core is introduced into it, it is understandable that it is necessary that the number of layers of fiber wound circumferentially is greater than or equal to the number of layers of fiber wound longitudinally when the outer sheath is made from longitudinal and circumferential fiber windings.

Reference can also be made to patent application WO 85/04380, which relates to devices for storing and transporting fluid under pressure. A composite material is wound helically on the cylindrical portions of these devices. This publication also shows that the thickness of the composite material can be variable. This can be achieved in particular by suitably controlling the winding machine so as to obtain a greater number of fiber layers in certain zones. As we will see from the description below, it can be interesting to obtain external wrappings having different thicknesses. The technique described can be used advantageously in this case. become.

As shown in Fig. 1, the outer casing 4 presents a frustoconical shape 11 at its end opposite to that intended to be inserted in the taper 3A of the base, which is fixed. This frustoconical shape 11 can be obtained by machining the outer casing after its manufacture. It can also be obtained directly during the manufacture of the casing, for example by making a thread winding on a mandrel having a frustoconical shape in a certain zone.

In the example shown in Fig. 1, the core also presents a frustoconical shape 12 at one of its ends. This is achieved in such a way that it is complementary to the frustoconical part 11 of the outer casing 4.

It will be understood that the presence of these complementary frustoconical shapes 11 and 12 facilitates the insertion of the core 6 into the outer casing 4. The frustoconical shapes are made on the outer casing 4 and the core 6 before the casing and the core are aligned by centering them substantially relative to each other, such that they are in the position shown in Fig. 1.

When the outer casing 4 and the core 6 are in this position, a preliminary traction is applied to the sliding rod 5 in the direction of the arrow by means of suitable traction means. A traction is thus applied to the core 6 in the direction of the casing 4 in such a way as to ensure a suitable seal between the parts in contact of each of the ends of the core 6 and the casing 4. As long as the traction is maintained, which allows the frustoconical shapes 11 and 12 to be kept in tight contact with each other, the The device for supplying a fluid under pressure is designed such that the fluid under pressure is injected into the longitudinal channel or channels 7. Via the radial channel or channels 8, the fluid under pressure is thus guided into the space 10 formed between the outer surface of the displaceable rod 5 and the inner surface of the outer casing 4.

As shown in more detail in Fig. 2, the fluid injected under pressure into the space 10 allows the outer portion to be confined to a zone of the core 6 and the inner portion to be expanded to a corresponding zone of the casing 4 until it is located above the outer portion of the core 6. A uniform tensile force is then applied to the core 6 in such a way that the core can be introduced into the corresponding zone of the casing 4. Then the fluid under pressure may confine the outer zone of the core 6 to the following zone and expand the inner portion of the casing 4 to the corresponding following zone to allow the core 6 to penetrate into the zone of the casing 4. Thus, the fluid under pressure allows the core 6 to be confined and the inner portion of the casing 4 to be enlarged as the core 6 is inserted into the casing 4; that is to say, the fluid under pressure acts progressively in the axial direction as the core 6 is displaced. It is understandable that the amount of tensile stress exerted on the sliding rod must be greater than the amount of forces opposing the penetration of the core into the casing, these forces resulting in particular from the friction of the core in the inner wall of the casing 4 and the pressure exerted on the frustoconical portion 12 of the core by the fluid under pressure.

In the example shown in Fig. 1, the outer casing 4 is made of composite material and the core 6 is metallic. It is therefore understandable that in this particular Case the introduction of the core into the casing is achieved solely thanks to the expansion of the casing made of composite material.

The process is also applicable to non-metallic cores and in particular to compressible cores, i.e. hollow cores. In this case, the injection of the fluid under pressure allows the core to be compressed. It also allows the outer casing to expand, provided that it is not solid.

The outer covering made of composite material has a certain expansibility. The expansibility can be adjusted depending on the materials desired to realize the outer covering by the above-mentioned method. In particular, when the outer covering is achieved by filamentary winding, the winding method used for each layer and the winding angles can be appropriately selected.

It has been stated above that the traction force exerted on the sliding rod 5 depends on the friction of the core on the inner wall of the casing 4. In order to reduce the friction forces and consequently reduce the traction force exerted on the rod, a fluid with lubricating properties can be chosen as the injected fluid. In particular, polymerizable synthetic resins can be selected for use.

It can also be noted that the force exerted by the fluid under pressure on the frustoconical shape 12 of the core 6 can be reduced, thus reducing the traction force exerted on the sliding rod 5. The reduction of this force due to the pressure is obtained by limiting as much as possible the difference between the diameter of the sliding rod 5 and the diameter of the core 6.

Once the core has been completely inserted into the external casing 4 made of composite material, the device producing the fluid under pressure is controlled so that the injection of this fluid through the sliding rod 5 stops, so that the casing 4, no longer subjected to internal pressure, can stop contracting radially around the core 6 and constricting it.

As can be seen from Fig. 4, the cutting then proceeds to the lost part of the casing 4, which is held by the base 1.

It will be understood that a thin film of fluid may exist between the inner surface of the sheath 4 and the core 6. This is why the use of a polymerizable plastic as a lubricating fluid may be particularly advantageous. In fact, after the core has been completely introduced into the sheath, the injection of fluid stopped and the sheath 4 has been completely contracted, it is possible to proceed to the polymerization of the remaining thin film of plastic present between the inner surface of the sheath and the outer surface of the core. This ensures the bonding of the core 6 to the inner wall of the sheath 4. The shrinking and the vacuum retention of the core under the sheath of composite material are thus enhanced.

The method according to the invention thus allows the expansion of the outer shell and/or the compression of the core. These changes in the diameter of the shell and the core can be quantified in the following way.

First, consider the general case in which the process according to the invention allows the expansion of the casing and the compression of the core simultaneously thanks to an internal pressure P₁ existing between the external surface of the core and the internal surface of the casing. Example of Figures 1 and 2, this pressure P₁ is the pressure of the fluid in the gap 10.

They are designated as:

Oo is the internal cross-section of the composite material casing at rest,

O'o is the internal cross-section of the enclosure subjected to the internal pressure P₁,

O''₀ is the internal cross-section of the cladding on the core.

The expansion factor K0 of the enclosure is:

K0; = O'o - O₀/O₀ P1;

Likewise are designated by:

O₂ is the outer cross-section of the core at rest,

O'₂ is the external cross-section of the core subjected to the external pressure P₁,

O''2 is the outer cross-section of the core surrounded by the outer cladding.

The compression factor K2 of the core is:

K2; = O'2; - O₂/O₂ P1;

When the casing is subjected to pressure P₁, the core is inserted when:

O'₀ > O'₂

The core is covered by the cladding if:

O₂ > O�0

After suppressing the pressure P₁, the inner cross-section of the cladding is fixed to the value O''₂ at O''₂ = 0''�min;₀ and exerts a shrinking effect on the core.

The composite material shell is thus held under tension and the core is held under compression.

The shrink pressure P at the transition point between the core and the outer casing is equal to:

P = O2 - O₀/(K₀O₀ - K₂O₂)

In the special case where the core is solid and therefore cannot be compressed, the process only allows the expansion of the outer shell made of composite material.

Thus, under the effect of pressure P₁, 0″�0 > O₂ must hold, and clamping occurs when O₂ > O₀.

If the nucleus is solid, one will obtain O₂ = O'₂ = O''₂ and K₂ = 0.

Consequently, the pressure at the transition point P is equal to

P = O₂ - O�0/Ko O�0 One can also consider the special case where the shell is solid and cannot be expanded, whereby the process allows the core to be compressed.

Also, under pressure P1, O'2 < O0 must hold, and sheathing will occur if O2 > O0.

If the shell is solid, we have O₀ = O'₀ = O''₀, and consequently the pressure at the interface between the core and the shell is equal

P = O2 - O₀/- K₂ O2;

In the example shown in Figures 1 and 2, the inner surface of the casing and the outer surface of the core are cylindrical. Moreover, the thickness of the outer casing is constant along its entire axis. Certainly, the method according to the invention can be applied not only to this Type of core and cladding can be applied.

This method is in fact applicable to a casing and a core in which the outer surface of the core and/or the inner surface and/or outer surface of the casing is a surface of revolution. Thus, the inner surface and/or outer surface of the casing and/or the outer surface of the core can be frustoconical. It is understandable that in this case it is not necessary to form frustoconical shapes at one end of the casing and on the core. It is even possible, according to the method of the invention, to cover a core having any shape with a casing which also has any shape.

It is further noted that in the example shown in Fig. 1, the inner surface of the sheath and the outer surface of the core are similar. The method according to the invention can also be used when these surfaces are not similar. It is understood that in this case the sheathing and vacuuming of the core is always ensured by the outer sheath by direct contact of the outer sheath on at least a portion of the periphery of the core.

It may also be advantageous to ensure the covering and the maintenance of the vacuum of the core by direct contact of the outer covering over its entire circumference. In this case, before aligning the core and the covering, provision may be made for the covering of the core to be made such that the inner surface of the covering and the outer surface of the covered core become similar.

The casing 4 can be held, possibly with a more or less significant clearance, by an external calibration tube of the casing. This stiffening tube 15, shown in Fig. 2, allows, for casings of low thickness, to avoid the problem of the formation of beads, jamming or deformation due to excessive friction during insertion.

The method according to the invention can also be carried out if the external enclosure is not a body of revolution. In this case, the expansion of the enclosure under the effect of the fluid injection under pressure is not uniform and deformations can occur. It is therefore advantageous to support the enclosure externally during the injection of the fluid under pressure by suitable means, not described in detail here, in order to limit its deformation.

For example, in Fig. 5, a cylindrical clad core 13 is shown held in place in an outer sheath 14, the inner surface of which is cylindrical but has a variable thickness.

As previously stated, processes are known that allow such a covering to be made of composite material by means of thread-like windings.

The casing 14 comprises, in general, three zones A, B, C, the thickness of zone A being greater than that of zone C, which in turn is greater than that of zone B. The thickness of this zone is chosen such that the casing 14 encases the core 13 and holds it under pressure, since the latter is subjected to greater stresses in zone A than in zone C, the stresses in zone C in turn being greater than those in zone B.

In the special case where the core is cylindrical and where the stresses to which it is subjected decrease from one of its ends to the other, it is possible in particular to use a casing whose inner surface is cylindrical and whose outer surface is frustoconical, the cross-section of this outer surface being from the end of the core, which is subjected to greater internal stresses, decreases towards the other end.

One can even imagine the special case in which the core is frustoconical and in which the stresses to which it is subjected decrease from the end of the weakest section to that which has the largest section. One can then use a casing whose inner surface is frustoconical and whose outer surface is cylindrical, the inner surface of the casing and the outer surface of the core being similar.

It is understood that an external sheath can be made according to any stresses to which the core is subjected by adapting its thickness, but also by using different materials according to the corresponding zones of the core.

For example, the method according to the invention makes it possible to coat a core at a pressure of approximately 1400 bar (1.4 x 10⁸ Pa) with a sheath of composite material which is dimensioned to have a breaking strength of 2000 bar (2 x 10⁸ Pa).

Finally, it can be noted that the method according to the invention allows the casing pressure to be adjusted with an accuracy of approximately ± 10%, the maximum value of the casing pressure being able to reach 70% of the compressive strength of the casing made of composite material.

The reference signs placed after the characterizing technical features in the claims are intended solely to facilitate the understanding of the latter and can in no case have the effect of limiting the invention to the particular embodiments described.

Claims (18)

1. Method for encasing and holding under pressure a core (6) by a sheath (4) made of composite material, the sheath (4) made of composite material having an inner cross-section which is smaller than the outer cross-section of the core (6) before the core is introduced, characterized in that that the procedure consists in: Holding one of the ends of the enclosure (4) by means of a base (1); Aligning the core (6) and the sheath (4) at their other end by substantially centering them relative to each other; Exerting a preliminary tensile force on the core (6) in the direction of the sheath (4) and, in the case of a hollow and axially open core (6), providing tight closure means such that an adequate seal is ensured between the contact area of each of the ends of the core (6) and the sheath (4); Injecting a fluid under pressure into the casing (4) and towards the core (6) to create a pressure between the core (6) and the casing and to progressively increase the cross-section of the casing (4) in the axial direction and possibly reduce the cross-section of the core (6), and at the same time continuously exerting a tensile force on the core (6) such that the core (6) is gradually introduced into the casing (4); Stopping the injection after complete insertion of the core (6) into the casing (4); Cutting off the excess portion of the casing (4) held by the base (1). 2. Method according to claim 1, characterized in that it additionally consists in making, before aligning and centering the sheath and the core, complementary conical shapes (11, 12) at the points of contact, the conical shape (11) being present at the end of the sheath (4) and the conical shape (12) being present at the end of the core (6), such that the centering and the appropriate seal between the core (6) and the sheath (4) is achieved by fitting the conical shapes (11) and (12) into one another. 3. Method according to claim 1, characterized in that the inner and/or outer surface of the envelope is a surface of revolution. 4. Method according to one of claims 1 or 3, characterized in that the outer surface of the core is a revolution surface. 5. Method according to one of claims 3 or 4, characterized in that at least one of the surfaces is cylindrical. 6. Method according to one of claims 3 - 5, characterized in that at least one of the surfaces is frustoconical. 7. Method according to one of claims 1 and 3 - 6, characterized in that the inner surface of the shell and the outer surface of the core are homothetic. 8. Method according to one of claims 1 and 3 - 6, characterized in that, if the inner surface of the casing and the outer surface of the core are not homothetic, it further consists in carrying out a covering of the core before aligning the core and the casing in such a way that the inner surface of the casing and the outer surface of the core have become homothetic. 9. Method according to one of the preceding claims, characterized in that the pressurized fluid is a lubricating fluid. 10. Process according to claim 9, characterized in that the pressurized fluid is a polymerizable resin. 11. Process according to claim 10, characterized in that it further consists in, after introducing the core into the casing and stopping the injection of the pressurized fluid, polymerizing the residual resin present between the inner surface of the casing and the outer surface of the core. 12. Method according to one of the preceding claims, characterized in that the covering is produced by rolling fibers impregnated with polymerizable resin onto a support and then polymerizing them. 13. Method according to claim 12, characterized in that the covering comprises at least one rolled-up fiber layer. 14. Method according to one of claims 12 or 13, characterized in that the thickness of the casing is variable. 15. Method according to one of claims 12 - 14, characterized in that the fibers are organic or mineral fibers with high mechanical resistance, such as fibers made of carbon, glass, silicon, boron or aramids. 16. Method according to one of the preceding claims, characterized in that the casing (4) is provided with a stiffening tube (15). 17. Method according to one of the preceding claims, characterized in that, when the casing is not a body of revolution, the method further consists in stiffening the casing during the injection of the pressurized fluid in order to limit its deformation. 18. Device for carrying out the method according to one of claims 1 to 17, characterized in that it comprises a sliding rod (5) of smaller diameter than the cross-section of the casing (4), which is intended to be connected to the core (6) and arranged in the casing (4) in such a way as to form a gap (10) between the outer surface of the rod (5) and the inner surface of the casing (4), the rod (5) also being provided with at least a longitudinal channel (7) and at least one radial channel (8), the longitudinal channel communicating with the intermediate space via the radial channel, that the device further comprises a system (9) which ensures the tightness between the casing (4) and the rod (5), means for exerting a tensile force on the rod (5) with respect to the casing (4) and a device which delivers a fluid under pressure, the device being connected to the longitudinal channel or channels to allow the fluid under pressure to be introduced into the intermediate space.